Thunder Ace brake problem

[Scratcher]

The Ace I acquired about two months ago has always had iffy brakes... definitely not the famed Yamaha 'Blue Spot' Stoppers.

When all of my stuff to give her a service had arrived I decided today was the day, so I start with a front brake bleed...

The fluid wasn't particularly grotty but brownish, the right hand caliper (looking at the front of the bike) bled as expected with the new fluid showing up in the pipe. I waited about 10 mins and rebled the right hand caliper but no bubbles.
I moved on the left one and repeated the process. Just after the clean fluid appeared in the pipe, the brake lever stopped resisting the pressure I applied. I quickly checked under the bike, the calipers, the banjo... no liquid.
I checked both bleed nipples were tight - So I have no leak and very little if any pressure to the calipers.

IS IT SAFE TO ASSUME THE MASTER CYLINDER SEALS HAVE GONE AND ARE ALLOWING FLUID TO MOVE AROUND IT? SHOULD I GET A MASTER CYLINDER REPAIR KIT?

Help on this would be greatly appreciated!

[nismo]

Are they the two piece (Thunderace) blue spots with a link pipe, or the later single piece units?

[YAMAEXUP]

Best to strip both the master cylinder (look for the large lump of gloop) & also the calipers. Remove the piston seals & clean behind where the seals sat in the ring land. Reassemble with proper seal grease & if you've got original brake lines in place , replace them with Goodridge or HEL lines.

Bleed the system with the master cylinder pointing downwards

[Crazy L]

Pointing downwards? I've not heard of that one, I'll have to give it a go next time I do my 'ace brakes.

I had a 'Cat too and they have always been a sod to bleed, just when you think no more bubbles could appear....."pop pop", lol.


Stonking brakes though once done, especially with braided hoses and EBC HH sintered pads in.